Zero valent iron/iron oxide mineral/ferrous iron composite for treatment of a contaminate fluid

ABSTRACT

The present inventors have discovered a novel composition, method of making the composition, system, process for treating a fluid containing a contaminant. The fluid may be aqueous. The contaminated fluid may be in the form of a suspension. The treatment reduces the concentration of the contaminant. The reduction in concentration of a contaminant may be sufficient so as to effect remediation of the fluid with respect to the contaminant. The treatment may reduce the concentration of a plurality of contaminants. The present composition, system, and process are robust and flexible. The composition includes zero valent iron, an iron oxide mineral, and ferrous iron. The ferrous iron promotes maintenance of the iron oxide mineral. The iron oxide mineral promotes the activity of the zero valent iron. The process and system may involve multiple stages. A stage may be optimized for treatment with respect to a particular contaminant. The present composition, system, and process are effective for treating a fluid containing one or more of a variety of contaminants such as toxic metals, metalloids, oxyanions, and dissolved silica. It may be applied to treating various aqueous fluids, such as groundwater, subsurface water, and aqueous industrial waste streams.

BACKGROUND

Wastewater treatment is one of the most important and challengingenvironmental problems associated with coal-based power generation.

Using wet scrubbers to clean flue gas is becoming more popular worldwidein the electrical power industry. In the coming years, hundreds of wetscrubbers will be installed in the US alone. While wet scrubbers cangreatly reduce air pollution, toxic metals in the resulting wastewaterpresent a major environmental problem. The industry prepares to investbillions of dollars in the next decade to meet more the ever morestringent environmental regulations; unfortunately, a cost-effective andreliable technology capable of treating such complicated wastewater isstill not available.

The compositions of FGD wastewaters vary greatly, depending not only onthe types of coal and limestone used but also on the types of scrubberand processes used. Pretreatment method and management practices alsoaffect wastewater characteristics. According to a recent survey by EPR1(2006), untreated raw FGD wastewater could have TSS in ˜10,000 mg/L butafter settlement, it falls to ˜10 mg/L; the pH typically falls in5.8-7.3; sulfate is in the range of 1,000-6,000 mg/L; nitrate-N at levelof 50 mg/L is not uncommon; chloride, alkalinity and acidity vary fromhundreds to thousands ppm; selenium exists in various forms, rangingfrom dozens of ppb to over 5 ppm, among which, selenate could accountfor about half of total Se; arsenic ranges from a few ppb to hundreds ofppb; mercury ranges from below 1 ppb to dozens of ppb; and boron can beas high as hundreds of ppm.

Treatment of selanate-Se in wastewater is often considered to be one ofthe most difficult in toxic metal treatments. Selenium is a naturallyoccurring chemical element in rocks, soils and natural waters. AlthoughSe is an essential micronutrient for plants and animals, it can be toxicat elevated levels and some of Se species may be carcinogenic. Thehexavalent selenium is stable in oxic environments and exists as theselenate (SeO₄ ²⁻) anion, which is weakly sorbed by mineral materialsand generally soluble. Tetravalent Se is the stable valence state undermildly reducing or anoxic condition (0.26 V<Eh<0.55 V at pH 7). Itexists as the selenite (SeO₃ ²⁻) anion, which tends to be bound ontomineral surfaces (e.g., Fe and Mn oxides). Selenate and selenite aremore toxic due to their high bioavailability than elemental selenium ormetallic selenides.

A biological treatment system, ABMet, has been patented and is beingmarketed by GE Water.

However, there remains a need for a cost-effective and reliabletreatment process for removing toxic pollutants from the wastewatergenerated by the wet scrubbers operated for flue gas desulfurization incoal-fired power plants.

SUMMARY

The present inventor has developed a chemical treatment process that cancost-effectively treat all major pollutants in the flue gasdesulfurization (FGD) wastewater in a single process.

The present inventor developed a fluidized reacting system using ahybrid reactive solid/secondary reagent reactor that cancost-effectively remove many toxic metals from wastewater. The systemand process are effective to treat an aqueous suspension. The systemuses a reactive solid and a secondary reagent as reactive agents torapidly reduce selenate to become insoluble selenium species, which arethen adsorbed or precipitated along with various of other toxic metals(such as As and Hg, if present) in wastewater onto the iron oxidesludge. The system is particularly effective for removing selenate-Se.

The present process is effective for removing almost all concern toxicmetals in an aqueous suspension; in addition, it can remove oxyanionpollutants and metalloids. More particularly, contaminants removable bythe present system and process are: most toxic metals such as arsenic,mercury, selenium, cobalt, lead, cadmium, chromium, silver, zinc,nickel, molybdenum, and the like; metalloid pollutants such as boron andthe like; many oxyanion pollutants, such as nitrate, bromate, iodate,and periodate, and the like; and the like.

The present system and process use common, non-toxic, and inexpensivechemicals. The present chemical treatment system costs much less toconstruct and operate than biological treatment systems, which tend tobe more complex.

The present system and process are versatile and flexible. The presentsystem and process are more robust and manageable than a biologicalprocess when exposed to toxic chemicals or any disturbances and changesin wastewater quality and quantity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustrating a single-stage fluidized bed reactor;

FIG. 2 is a flow chart illustrating a three-stage reaction system;

FIG. 3 is a schematic illustrating a single-stage fluidized bedZVI/FeOx/Fe(II);

FIGS. 4A, 4B are pictures illustrating a bench scale single-stagereactor;

FIGS. 5A, 5B are pictures illustrating an alternative bench scalesingle-stage reactor;

FIGS. 6A. 6B, 6C are pictures illustrating a bench scale three-stageZVI/FeOx/Fe(II) fluidized-bed reactor system; and

FIG. 7 shows three panels of pictures illustrating settling of a mixtureof Fe⁰ and magnetite powder rich of surface bound Fe(II); pictures takenafter settling for 1 min (left panel), 3 min (middle), and 6 min(right).

DETAILED DESCRIPTION

The present inventors have discovered a novel system for treatingwastewater. Experiments have demonstrated the system operable forremoval of selenium present as selenate.

According to some embodiments, a reactor system includes zero valentiron. According to some embodiments, ferrous iron is added to a reactorsystem. The present inventor believes that ferrous iron acts as apassivation reversal agent for zero valent iron. The mechanism iscomplex. While not wishing to be limited by theory, the present inventorbelieves that passivation is partially caused by corrosion of iron in awater environment. The present inventor believes that ferrous iron actsto cause conversion of iron corrosion product on the surface of the zerovalent iron to magnetite. According to some embodiments, a sufficientamount of magnetite is produced so as to optimize removal of toxicmaterials by a reaction system including zero valent iron. According tosome embodiments, the process produces removable solids. According tosome embodiments, the removable solids contain toxic materialencapsulated in magnetite. According to some embodiments, theencapsulated toxic material is solid.

Thus, according to some embodiments, the process uses a highly reactivemixture of zerovalent iron (Fe⁰), iron oxide minerals (FeOx), andferrous iron (Fe^(II)) to react with, absorb, and precipitate varioustoxic metals and metalloids from wastewater, forming chemically inertand well crystallized magnetite (Fe₃O₄) particles that can be separatedfrom water and disposed with encapsulated pollutants.

According to some embodiments, the reactive zone is maintained nearneutral pH.

The present inventor believes that boron in the wastewater furthercontributes to passivation and that ferrous iron removes boron form thezero valent iron.

It will be understood that wastewater is illustrative of an aqueoussuspension. For example, the present inventor contemplates treating oilrefinery waste. Further, the present inventor contemplates treatingwetlands.

It will be understood that selenium is illustrative of a toxic material.Other common toxic materials are contemplated. For example, the presentinventor contemplates removing arsenic, mercury, cobalt, lead, cadmium,chromium, silver, zinc, nickel, molybdenum, and the like; metalloidpollutants such as boron and the like; many oxyanion pollutants, such asnitrate, bromate, iodate, and periodate, and the like; and the like.

It will be understood that ferrous iron is illustrative of a secondaryreagent. The secondary reagent is desirable adapted to act as apassivation reversal agent. Passivation is generally the process ofrendering an active material, for example zero valent zinc, inactive.Aluminum ion, Al³⁺, may substitute for (e.g. added as aluminum sulfate)for ferrous iron. It will be understood that iron is illustrative of areactive solid. The present inventor believes that iron is particularlypractical. However, the present inventor contemplates other treatmentmaterials. For example, according to some embodiments, the treatmentmaterial is zinc. It will be understood that a reactive system mayinclude the treatment material in zero valent form. According to someembodiments, the reactive system further includes a passivation reversalagent suitable for the zero valent form as may be advantageous.

According to some embodiments, a reactor includes an internal settlingzone in communication with a reactive zone. The reactor is illustratedin schematic in FIG. 1. According to some embodiments, the internalsettling zone uses gravitational forces to separate solids from liquids.According to some embodiments, mostly liquids remain in the settlingzone. According to some embodiments, the internal settling zone istowards the top of the reactor (FIG. 1). According to some embodiments,communication with the reactive zone is via an inlet at the bottom ofthe internal settling zone. According to some embodiments, effluent isremoved from the top region of the internal settling zone. According tosome embodiments, the effluent is very clear. Magnetite is known to beblack. Settling observed in an experiment over time is illustrated inFIG. 3 of the document “pilot test scale plan” appended hereto, whichshows clearer separation of black material and clear fluid over time.The present inventor believes that settling for a separating method isparticularly efficient. However, other suitable separating methods arecontemplated.

According to some embodiments, a reactive zone includes a centralconduit. The central conduit improves mixing. For example, according tosome embodiments, the central conduit promotes convective motion.

Thus, according to some embodiments, the reactor system operates as afluidized bed that employs a motorized stirrer in conjunction with acentral flow conduit to create a circular flow within the reactor andprovide an adequate mixing between reactive solids and wastewater. Aninternal settling zone was created to allow solid-liquid separation andreturn of the solid into the fluidized zone.

FIG. 1 is a schematic illustrating an embodiment of the system andprocess. A single-stage fluidized-bed system includes a fluidizedreactive zone, an internal solid/liquid separating zone, an aeratingbasin, a final settling basin, and an optional sand filtration bed.

Still referring to FIG. 1, the fluidized zone is the main reactive spacewhere reactive solid, in the form of particles, is completely mixed withwastewater and secondary reagent and where various physical-chemicalprocesses responsible for toxic metal removal occur.

Still referring to FIG. 1, the internal settling zone is to allowparticles to separate from water and be retained in the fluidized zone.For high density particles, an internal settling zone with a shorthydraulic retention time is sufficient for complete solid/liquidseparation. This eliminates the need of a large external clarifier and asludge recycling system.

Still referring to FIG. 1, the aeration basin has two purposes: (1) toeliminate residual secondary reagent in the effluent from fluidizedzone; and (2) to increase dissolved oxygen level. For a single-stagereactor, effluent from fluidized reactive zone will always containcertain amount of secondary reagent. Oxidation of secondary reagent willconsume alkalinity and therefore will lower the pH. To accelerateoxidation of secondary reagent, the aeration basin should maintain a pHof above 7.0. Chemicals such as Ca(OH)₂, NaOH and Na₂CO₃ could be usedfor pH control.

Still referring to FIG. 1, the final settling tank is to removeflocculent formed in the aeration basin. The floc (fluffy) settled tothe bottom can be returned to the fluidized zone and transformed bysecondary reagent into dense particulate matter.

Still referring to FIG. 1, upon final settling, a sand filtration bedmay be used to further polish the treated water before discharge.

Still referring to FIG. 1, the post-FBR (fluidized bed reactor) stages(aeration-settling-filtration) may not be needed under certain operationconditions.

Referring now to FIG. 2, several fluidized-bed reactors can be combinedto form a multi-stage treatment system. It is recommended that eachstage maintain its own reactive solid. That is, the solids are separatedin each stage. In order to achieve a separate solid system, each stagemay have its own internal solid-liquid separation structure.

Still referring to FIG. 2, depending on operating conditions in theFBRs, the wastewater characteristics, and discharge standards, the postFBR treatments (aeration+final clarifier+sand filtration) may not beneeded.

Although a multi-stage system is more complex and may result in a higherinitial construction cost, a multi-stage fluidized-bed reactor systemhas several major advantages.

A multi-stage system can achieve higher removal efficiency than asingle-stage system under comparable conditions. Further, the FGDwastewater may contain certain chemicals (i.e., phosphate) that may bedetrimental to the high reactivity of the reactive solids. A multi-stagesystem can intercept and transform these harmful chemicals in the firststage and thus reducing the exposure of the subsequent stages to thenegative impact of these chemicals. As such, a multi-stage configurationis more stable and robust.

A multi-stage configuration facilitates the control of nitratereduction, for example in an iron-based system. In a single stagesystem, because the presence of dissolved oxygen carried in rawwastewater, it tends to be difficult to operate the system in a rigorousanaerobic environment. In a multi-stage system, stage 1 can removevirtually all dissolved oxygen; as a result, the subsequent stages canbe operated under rigorous anaerobic environment.

A multi-stage system allows flexible control of different chemicalconditions in each individual reacting basin. The chemical conditions ineach reactive basin can be controlled by adjusting the pumping rate ofsupplemental chemicals and turning aeration on or off. A multi-stagesystem can be operated in a mode of multiple feeding points. Each stagemay be operated under different pH and dissolved oxygen condition.

A multi-stage system will lower chemical consumption. In a single-stagecomplete-mixed system, secondary reagent in the reactor are desirablymaintained at a relatively high concentration in order to maintain highreactivity of reactive solids. As a result, the residual secondaryreagent in the effluent will be high. This means that more secondaryreagent will be wasted and more NaOH (or lime) consumption will berequired just to neutralize and precipitate the residual secondaryreagent in the effluent. As a result, more solid sludge will be producedand waste disposal cost will increase. In a multi-stage system, residualsecondary reagent from stage 1 can still be used in stage 2. In thiscase, secondary reagent can be added in a way that conforms to itsactual consumption rate in each stage. As a result, it is possible tocontrol residual secondary reagent in the effluent in the final stage tobe much lower than the one in a single stage system.

Referring to FIG. 3, according to some embodiments, in the system andprocess illustrated by FIG. 1, the reactive solid includes zero valentiron (ZVI) and iron oxide mineral (FeOx), and the secondary reagent isFe²⁺. Thus, referring to FIG. 3, a single-stage fluidized-bedZVI/FeOx/Fe(II) system includes a fluidized reactive zone, an internalsolid/liquid separating zone, an aerating basin, a final settling basin,and an optional sand filtration bed.

Still referring to FIG. 3, the fluidized zone is the main reactive spacewhere ZVI and FeOx reactive solids are completely mixed with wastewaterand dissolved Fe²⁺ and where various physical-chemical processesresponsible for toxic metal removal occur.

Still referring to FIG. 3, the internal settling zone is to allow ZVIand FeOx to separate from water and be retained in the fluidized zone.Because of high density of fully or partially crystallized FeOxparticles, an internal settling zone with a short hydraulic retentiontime would be suffice for complete solid/liquid separation. Thiseliminates the need of a large external clarifier and a sludge recyclingsystem.

Still referring to FIG. 3, the aeration basin has two purposes: (1) toeliminate residual dissolved Fe²⁺ in the effluent from fluidized zone;and (2) to increase dissolved oxygen level. For a single-stage reactor,effluent from fluidized reactive zone will always contain certain amountof dissolved Fe²⁺. Oxidation of Fe²⁺ will consume alkalinity andtherefore will lower the pH. To accelerate oxidation of dissolved Fe²⁺,the aeration basin should maintain a pH of above 7.0. Chemicals such asCa(OH)₂, NaOH and Na₂CO₃ could be used for pH control.

Still referring to FIG. 3, the final settling tank is to remove ironoxide flocculent formed in the aeration basin. The ferric oxide floc(fluffy) settled to the bottom can be returned to the fluidized zone andtransformed by Fe²⁺ into dense particulate matter.

Still referring to FIG. 3, upon final settling, a sand filtration bedmay be used to further polish the treated water before discharge.

Still referring to FIG. 3, the reactive solid may initially be zerovalent iron, with the iron oxide mineral formed in situ. The iron oxidemineral may coat the zero valent iron.

Still referring to FIG. 3, the system can be operated under variouscontrolled conditions as needed.

According to some embodiments, an iron-based technique employs a mixtureof zerovalent iron (ZVI or Fe⁰) and iron oxide minerals (FeOx), andFe(II) species to react with, adsorb, precipitate, and remove varioustoxic metals, metalloids and other pollutants from the contaminatedwastewater. According to some embodiments, an iron-basedphysical-chemical treatment process that employs a hybrid ZerovalentIron/FeOx/Fe(II) Reactor to treat toxic metal-contaminated wastewater.For example, according to some embodiments, the present system andprocess involve a hybrid Zerovalent Iron/FeOx/Fe(II) reactor forremoving toxic metals in wastewater. According to some embodiments, theprocess employs a fluidized bed system and use a reactive mixture ofFe⁰, Fe^((II)) and FeOx to absorb, precipitate, and react with varioustoxic metals, metalloids and other pollutants for wastewaterdecontamination. According to some embodiments, toxic metals areencapsulated within iron oxide crystalline (mainly magnetite powder)that are chemically inert and physically dense for easier solid-liquidseparation and final disposal.

While not wishing to be limited by theory, the present inventor believesthat the following are contributing mechanisms for the present ironbased system and process: a) using the reducing power of Fe⁰ and Fe(II)to reduce various contaminants in oxidized forms to become insoluble ornon-toxic species; b) using high adsorption capacity of iron oxidesurface for metals to remove various dissolved toxic metal species fromwastewater; and c) promoting mineralization of iron oxides and growth ofcertain iron oxide crystalline so that surface-adsorbed or precipitatedtoxic metals and other pollutants could be incorporated into iron oxidecrystalline structure and remain encapsulated in a stabilized form forfinal disposal.

EXAMPLES Experimental Results of Using a Hybrid ZVI/FeOx/Fe(II) ReactiveSystem to Treat FGD Wastewater

The present system and process are a result of laboratory researchconducted by the present inventor to develop a cost-effective method forremoving toxic metals in the flue gas desulfurization wastewatergenerated from wet scrubbers of coal-fired steam electric power plants.Although developed specifically for treating the FGD wastewater withselenium as the main target contaminant, this chemical reactive systemis suitable for general application of removing a wide spectrum of toxicmetals in industrial wastewater, tail water of mining operations, andcontaminated groundwater.

According to various experimental embodiments, as shown herein, a singlestage may achieve 90% selenate removal within 4 hr reaction time. Athree-stage system, in comparison, may achieve a 96% removal rate.

The present inventor believes that some exemplary novel aspects are:

-   -   1) Discovery of the role of externally-added Fe²⁺ in sustaining        the reactivity of Fe⁰ with respect to selenate reduction.        Externally-added Fe²⁺ may convert less reactive ferric oxide        coating on Fe⁰ particles into a highly reactive mix-valent Fe₃O₄        oxide coating and therefore rejuvenate the passivated Fe⁰        surface.    -   2) Discovery that surface-bound Fe(II) on magnetite (Fe₃O₄)        particles can rapidly reduce selenate to insoluble elemental Se        and be removed from the liquid phase.    -   3) Discovery that the chemical conditions that promote the        formation of magnetite (Fe3O4) as a reaction product from the        oxidations of Fe0 and surface-bound Fe (coupled with reductions        of dissolved oxygen, nitrate, and selenate in the water).    -   4) Development of a fluidized bed system with an internal        settling zone and a central conduit that can (a) retain high        concentration of Fe₃O₄ solid particles and therefore offer        abundant reactive surface area that can host surface bound        Fe(II)-selenate redox reaction; (b) offer an effective mixing        condition so that Fe⁰, Fe₃O₄ and s.b.Fe(II) can achieve their        respective roles in removing toxic metals; (c) avoid excess        diffusion of oxygen from air into the reactive system so that        less Fe⁰ and Fe(II) are wasted.    -   5) Development of a multiple-stage fluidized bed system that        will (a) achieve better toxic metal removal efficiency; (b)        control nitrate reduction efficiency to a level of desire; (c)        reduce consumption of ferrous salt and Fe⁰; (d) reduce or        completely eliminate residual dissolved Fe²⁺.

Bench Scale Tests Single Stage Reactor

Three Bench-Scale Fluidized-Bed Reactors were Fabricated and Operated.

Referring to FIGS. 4A and 4B, Reactor#1 has an internal settling zone(the compartment on the left side) in which it allows reactive solid toseparate from the water and be retained within the fluidized zone.Reactor#2 (not shown) is identical to Reactor#1. Reactor#1 and #2 bothhad an operating capacity of 7.2 L and had an internal settling zone(0.5 L) within the reactors (FIGS. 4A and 4B).

Referring to FIGS. 5A and 5B Reactor#3 is an integral system that has aninternal settling zone (far left), an aeration basin (near left), and asecond settling basin (right) within the reactor. Reactor#3 had anoperating capacity of 10 L. It had a built-in aeration basin (0.6 L) anda built-in final settling basin (FIGS. 5A and 5B). Peristaltic pumps(Masterflex pumps, Cole-Parmer, Illinois) were used to pump inwastewater and the needed chemical reagents. A small aquarium air pump(purchased from Wal-Mart) as used to provide aeration. A motorizedstirrer (max. 27 watt, adjustable rpm 100-2000, three-blade propellerstirrer) was used to provided mixing condition.

Zerovalent iron powder used in the tests was obtained from HepureTechnology Inc., including I-1200+ and HCl5 (see Batch Test results formore details). Other reagents used in the operation include HCl, FeCl₂,and NaOH.

Start Up

Contrary to what many experts in ZVI technology believed, fresh ZVI doesnot tend to be effective for chemical reduction of selenate. Batch testresults (Appendix B and Appendix C) confirmed that ZVI grains coatedwith magnetite could achieve a much higher reaction rate than ZVI grainsof a relative fresh surface with little or very thin iron rusts. Toimprove performance of a ZVI system, a unique start-up process isemployed to coat the ZVI powder surface with a more reactive andpassivation-resistant, chemically-stable magnetite coating. When areactor was started with using fresh ZVI powder, it took some time undercarefully controlled chemical environment to coat ZVI with a magnetitelayer.

Several factors are desirably considered in order to have a rapid andsuccessful start-up for a treatment system. First, the physical chemicalproperties of iron, most important the size distribution of ironparticles, are considered. Both reductions of selenate by ZVI and bysurface bound Fe(II) (s.b.Fe(II)) on magnetite are surface-mediatedheterogeneous reaction; therefore, increasing solid-liquid interfacialarea would increase overall reaction rate. Fine ZVI powders couldprovide larger surface area and therefore achieve higher selenatereduction under comparable conditions. This was confirmed in batchtests. The continuous flow reactor tests were successfully started upfive times. It appears that finer iron particles (dominant size: <45 μmin diameter) may be started up faster than larger particles (dominantsize: 45-150 μm in diameter). The chemical purity of ZVI powder wasfound to not a major factor. In batch and continuous-flow tests, variouspurities and composition of ZVI powder were used. No major differenceswere observed among the different iron sources with respect to reactionmechanism and rate for selenate reduction. Overtime, the zerovalent irongrains may all be coated with a magnetite coating and in the present ofdissolved Fe²⁺, they all achieve high reactivity for selenate reduction.

Generation of a magnetite coating on a ZVI particle is helpful to thesuccess of the system. Appropriate aqueous chemical conditions must bemaintained for the purpose. Iron corrosion could produce various ironoxides under different chemical conditions. Our batch and continuousflow reactor tests show that in order to generate magnetite from ironcorrosion reaction, three conditions must be met: a pH of 6.5 to 7.5;adequate dissolved Fe²⁺ that can form s.b.Fe(II); and appropriatespecies and concentration of oxidants. Oxidants can be certain oxyanionssuch as selenate, nitrate, nitrite, iodate (IO₃ ⁻) and periodate (IO₄ ⁻)in the wastewater. Oxidation of ZVI by these oxidants tends to formferric oxides (most likely lepidocrocite, γ-FeOOH). The small quantityof ferric oxides can be transformed to magnetite in the presence ofsurface-adsorbed Fe(II). Dissolved oxygen can also serve as an oxidantto generate magnetite (Huang et al. 2006). Low-intensity aeration in theearly stage could accelerate the magnetite-coating process.High-intensity aeration should be avoided because it could form largequantity of ferric oxides even in the presence of dissolved Fe²⁺ andmoreover, it will waste ZVI. Our experiences from live successfulstart-ups using simulated FGD wastewater indicates that in general thesystem will take about one to two weeks for the fresh ZVI to mature;over time, the system will gradually improve before reaching a state ofhigh performance.

As an alternative (and recommended) start-up procedure, we used nitratesolution (add 30 mg/L nitrate-N in tap water, operating HRT=12 hr)instead of simulated FGD wastewater to feed the system. Nitrate would becompletely reduced and in the presence of adequate dissolved Fe²⁺, ahigh quality (better crystallized and less amorphous, containing lessferric oxides or ferrous hydroxides) magnetite coating can be formed onZVI particles. Start-up with nitrate solution would take only two days.

A general start up procedure and exemplary controlled parameters are:

-   -   1) Select ZVI sources. Finer iron powder (<50 μm) is preferred.        Low iron purity and rusty surface in general are not a problem.    -   2) Add 80-100 g/L ZVI powder in the fluidized zone. Turn on        mixing equipment.    -   3) Start-up with FGD wastewater        -   Feed FGD wastewater at a rate equivalent to HRT=12 hrs. The            exact compositions of raw FGD wastewater may vary widely,            but in general contains high concentration of Cl⁻, sulfate,            and relative high concentration of nitrate.        -   Feed FeCl₂ solution (0.1 M FeCl₂ in 0.005 M HCl solution) at            a rate equivalent to 1.5 in mole Fe²⁺ per 1 L wastewater        -   Feed NCl at a rate to control the pH in the fluidized zone            at 6.8-7.2.        -   If the FGD wastewater contains limited concentration of            nitrate (e.g., below 10 mg/L nitrate-N), then a low            intensity aeration in the fluidized bed should be provided            to accelerate the formation of a magnetite coating.        -   Start-up with nitrate solution        -   Feed nitrate solution (30 mg/L nitrate-N) at a rate            equivalent to HRT=12 hrs.        -   Feed FeCl₂ solution (0.1 M FeCl₂ in 0.005 M HCl solution) at            a rate equivalent to 1.5 in mole Fe²⁺ per 1 L wastewater        -   Adjust HCl solution (0.1 M HCl) feeding rate to control the            pH in the fluidized zone at 7.0-7.5.

Normal Operation

Once started up successfully, the system requires only low-levelmaintenance effort. Routine operations and maintenances include one ormore of:

-   -   (a) Monitor the quality of wastewater entering the system. The        most important parameters include: pH, alkalinity, acidity,        total suspended solid (TSS). Of course, toxic constituents in        the raw wastewater should be monitored.    -   (b) Monitor the pH in the fluidized reactive zone. Performance        of the system depends mostly on pH. For a single-stage system,        pH in the reactive zone should be maintained within 6.5 to 7.5.        Both HCl and FeCl₂ can be used to control the system.    -   (c) Monitor the pH in the aeration basin. Dissolved Fe²⁺ can be        oxidized more rapidly at pH>7.0. Formation and settling        properties of ferric oxide flocculent depends also on pH.        Therefore, it is recommended that aeration basin be operated at        pH 7.5-8.0.    -   (d) Monitor the performance of settling tank and sand filtration        bed. The maintenance requirements are no different from those        unit processes in typical water or wastewater treatment plants.        Most importantly, the settled sludge should be discharged or        returned at an appropriate rate to avoid excessive build-up of        the reactor.    -   (e) Excess solid discharge and disposal.

If the raw wastewater contains relative high suspended solids, apre-settling basin may be needed to reduce TSS in wastewater beforeentering the system. This can avoid accumulation of inert TSS in thefluidized reactive zone that might dilute the effective ZVI/FeOx solidconcentration.

For a single-stage reactor, the concentration of total solid in thefluidized zone could be maintained between 80 and 120 g/L. Assuming that30 mg Fe²⁺/L be converted to Fe₃O₄ and the reactor is operated at HRT=4hours (based on test results), we estimate that it will add 0.25 g/LFeOx solid per day and therefore will take 160 days for the reactor toincrease its solid from 80 g/L to This estimate conform to the fact thatduring a three-month continuous flow test (hydraulic retention timevaries between 3 to 12 hours), we discharge no solid from the fluidizedbed reactor.

ZVI/FeOx reactive solids are considered mature when the surface of ZVIgrains is covered with well crystallized magnetite (dark black colorafter dry) and a significant presence of discrete magnetite crystalline(may be aggregated into a larger particle due to its strong magneticproperty). Unlike typical ZVI powder, matured ZVI/FeOx reactive solidswill not cement easily when settled at the bottle. Therefore, thereactor could be stopped for weeks with no risk of iron powdercementation. That is, the reactor can be stopped and restarted veryflexibly without a need to vacate the ZVI/FeOx mixture from the reactor.

Results

Results of testing are described in Appendix A and Appendix D. Theresults demonstrate that a single-stage reactive system alone caneffectively remove high concentration of selenate within a relativelyshort reaction time. A multiple-stage system can further improve theoverall performance. Since for most FGD wastewater, Se(VI) concentrationwill be lower than 5 mg/L used in this test (most typically, 1-2 mg/L),the present inventor estimates that an HRT of less than 4 hours would besufficient for most applications. Moreover, the reactor is operated atnear neutral pH.

Multi-Stage Reactor

The start-up procedure and normal operation requirements described for asingle-stage system can be similarly applied for a multistage system.Again, it is desirable that nitrate solution be used for rapid start-up.Nitrate solution was also found to be very effective in rejuvenating afouled system in which the system was accidentally acidified (pH droppedto below 4.0) for a few hours, which might permanently damage iron oxidereactivity and resulted in extremely poor performance even afterreturning to normal operation conditions.

Referring to FIGS. 6A, 6B, 6C, In this test, Reactor#1, #2, and #3 wascombined in sequence to form a three-stage FBRs treatment system. Thissystem was a 24-liter three-stage ZVI/FeOx/Fe(II) fluidized-bed reactorsystem. Initial testing on the three-stage system is described inAppendix A and Appendix D.

Continuous flow tests were conducted for six months on the bench-scale(24 liter) three state fluidized bed system based on the ZVI/FeOx/Fe(II)technique with high-strength raw FGD wastewater (provided by SouthernCompany).

The system was demonstrated during a 6 month testing period to be acomplete success, as shown in Table 1.

TABLE 1 Major Concentration in Concentration after Removal PollutantsFGD wastewater treatment Efficiency Selenium 7.8 mg/L SeO₄ ²⁻—Sedissolved Se   >98% <0.15 mg/L Mercury 335 μg/L dissolved Hg dissolvedHg >99.9% <0.2 μg/L Arsenic* 400 μg/L dissolved dissolved As >99.9%As(III) and As(V) <0.2 μg/L Nitrate 26 mg/L nitrate-N nitrate-N   >80%<5.0 mg/L Boron** 200~600 mg/L projected to be dissolved B >70% Notes:*The original raw FGD wastewater provided by Southern Co. contains onlyless than 0.6 μg/L total dissolved As. To evaluate Arsenic treatmenteffectiveness, 400 μg/L arsenite-As and arsenate-As was added. **Removalof dissolved boron in the system is still being tested and needs to befurther verified.

Laboratory Tests

Extensive laboratory tests have been conducted to understand thetreatment conditions and mechanisms.

Referring to FIG. 7, settling of reactive solid (black) from fluid(clear) is illustrated.

This inventor has conducted extensive batch tests (Appendix B, AppendixC, and Appendix D) in addition to the continuous flow tests (Appendix Aand Appendix D) to investigate the fundamental chemistry and applicationissues in the complicated reactive system that comprised of Fe⁰,dissolved Fe²⁺, various FeOx in different forms and compositions,dissolved oxygen, simulated FGD wastewater or real FGD wastewater with avery complex matrix of constituents. Laboratory experiments and theirresults are described in details and discussed in depth in the appendeddocuments. Findings from these tests are summarized as below:

-   -   1) In a rigorous anaerobic condition, selenate (at ppm level        concentration) cannot be effective reduced by pure Fe⁰ (with        fresh surface that contains negligible iron oxides). Only        negligible selenate could be reduced. That is, reactivity of Fe⁰        will be naturally passivated by the presence of selenate. This        explains why previous investigators failed to achieve a        sustainable removal when using Fe⁰ to reduce selenate.

SeO₄ ²⁻+2Fe⁰+2H₂O→Se⁰↓+2FeOOH+2OH⁻  (eq. 1)

-   -   -   Lepidocrocite (γ-FeOOH) forms a passive coating on the            surface of Fe⁰ particle and therefore inhibits further            reaction between Fe⁰ and selenate.

    -   2) In the presence of dissolved oxygen, selenate could be        reduced by Fe⁰ at a modest rate; however, to sustain the desired        selenate-Fe⁰ reaction, much of Fe⁰ will be wastefully consumed        by dissolved oxygen as a result. The implication is: An        excessive aerated Fe⁰ system might be able to remove selenate,        but the process is economically infeasible due to wasteful        consumption of Fe⁰ by oxygen and generation of large quantity of        iron oxide sludge.

    -   3) Reduction of selenate could be greatly accelerated in the        presence of dissolved Fe²⁺ at circum-neutral pH environment. The        reaction rate increases as dissolved Fe²⁺ increase. A presence        of 0.3 mM dissolved Fe²⁺ will be adequate. At near neutral pH        and anaerobic environment, the reaction will form magnetite as        their product.

SeO₄ ²⁻+2Fe⁰+Fe²⁺→Se(0)↓+Fe₃O₄  (eq. 2)

In this reaction, the direct role of Fe²⁺ might be to facilitating theconversion of passive FeOOH to reactive Fe₃O₄ and therefore, greatlyaccelerating the reaction.

-   -   4) Selenate could be rapidly reduced by s.b.Fe(II) on activated        magnetite surface at near neutral or weak acidic pH in the        absence of Fe⁰.

SeO₄ ²⁻+9s.b.Fe^((II))→Se(0)↓+3Fe₃O₄+2OH⁻  (eq. 4)

-   -   -   Unlike Fe²⁺ in the equation 2, Fe(II) here serves as a            reductant and directly contributes one electron to the            reduction of selenate.

    -   5) Nitrate, which is often present at tens of ppm level in the        FGD wastewater, will not inhibit selenate reduction by Fe⁰.        Indeed, nitrate was found to slightly accelerate selenate        reduction by Fe⁰. In contrast, reduction of nitrate by Fe⁰ will        be inhibited by the presence of selenate. In a rigorous        anaerobic environment, reduction of nitrate by Fe⁰ can occur        only after selenate is completely reduced in the system.

    -   6) Both reductions of nitrate and selenate by Fe⁰ will consume        certain amount of Fe²⁺. Nitrate reduction consume 0.75 mM        Fe(II)/1.0 mM nitrate; selenate reduction consume approximately        1.0 mM Fe(II)/1.0 mM selenate.

    -   7) The complex matrix of constituents in the FGD wastewater may        affect selenate reduction rate in the Fe⁰/FeOx/Fe(II) system.        Sulfate will slow down the reaction rate several folds. Chloride        at a lower concentration

    -   8) Source of Fe⁰. The mechanisms of Fe⁰-selenate reaction will        not be altered by the use of difference Fe⁰ sources. Tests with        different purities of Fe⁰ show that Fe⁰ purity has no apparent        relationship with the achievable reaction rate. There is no        obvious advantage from the use of high pure (>99%), little        rusted, electrolytic iron powder (Fisher Scientific) over        low-grade (95%), industrial iron filings. The size of iron power        however does matters. Fine iron powder will provide more        reactive surface than coarse iron powder. Fine iron powder may        also mature faster and ease start-up of the system.

Pilot Scale Tests Prophetic Example

The success of the laboratory-scale prototype has paved the road forconstructing a pilot-scale system and conducting extended fielddemonstrations to further evaluate, develop and refine the technology.

The present inventor contemplates a pilot-scale treatment system basedon a proved laboratory-scale prototype and conduct long-term field teststo further develop the technique and finalize its design forcommercialization.

The pilot scale test may involve one or more steps, such as: design andconstruct a pilot treatment system based on the laboratory prototype;conduct on-site long-term demonstrations in conjunction with furtherlaboratory mechanistic study; collaborated closely with industry andother stakeholders to further refine the system to meet the industrialneeds and environmental goals. Contemplated pilot scale tests arefurther described in Appendix D.

The present inventor contemplates an integral treatment system that cantreat FGD wastewater at a flow rate of 2 to 5 gallon per minute, whichrepresents about 1% of wastewater expected from a 1,000 megawatt powerplant. The pilot system may be mounted on a trailer that is adapted tobe hauled to different test sites.

Industrial Operation Prophetic Example

Based on the bench scale test described above, the present inventorestimates that for treating a 500 gpm FGD waste stream from a 1,000megawatt, a iron-based system will consume per year: 200 to 400 ton ofiron chemical (est. bulk price: $1,000 to $2,000/ton); 200 to 400 tonsof concentrated HCl; 50-200 kilowatt electric power consumption.Further, the present inventor estimates that for treating a 500 gpm FGDwaste stream from a 1,000 megawatt, a iron-based system will generateper year: 300 to 800 tons of iron oxide (weight in dry mass; laden withtoxic metals), to be disposed as a hazardous waste.

Chemical Consumptions

For treating 1 m³ FGD wastewater of typical strength, the system willconsume:

-   -   100-300 g ZVI (Fe⁰) powder    -   50-120 g iron salt    -   20-100 g NaOH (or equivalent amount of Ca(OH)₂)    -   <0.2 L concentrated HCl    -   The total chemical cost will be less than $1.0 per 1 m³ FGD        wastewater. The system will produce 0.5-1.0 kg waste solid per 1        m³ wastewater treated. A 1,000-megawatt power plant may produce        1,000 to 3,000 m³ FGD wastewater per day (approximately 200-600        gpm) depending the specific operation conditions of the wet        scrubbers.

1. A treatment system for treating an aqueous suspension, wherein thetreatment system comprises a chemical reactor system comprising afluidized bed reactor comprising a reactive zone.
 2. The treatmentsystem according to claim 1, wherein the chemical reactor system furthercomprises an internal settling zone in communication with the reactivezone.
 3. The treatment system according to any one of claims 1-2,wherein the internal settling zone is located in the top region of thechemical reactor system.
 4. The treatment system according to any one ofclaims 1-3, wherein the internal settling zone comprises an opening atthe bottom of the internal settling zone adapted for the communicationwith the reactive zone.
 5. The treatment system according to any one ofclaims 1-4, wherein the internal settling zone comprises an outletadapted for removal of effluent from the internal settling zone.
 6. Thetreatment system according to any one of claims 1-5, wherein thereactive zone comprises a conduit.
 7. The treatment system according toclaim 6, wherein the conduit is central with respect to the reactivezone.
 8. The treatment system according to any one of claims 1-7,wherein the treatment system is a multi-stage system comprising anadditional reactor system.
 9. The treatment system according to any oneof claims 1-8, wherein the reactive zone comprises a reactive solid anda secondary reagent.
 10. The treatment system according to claim 9,wherein the reactive solid comprises iron.
 11. The treatment systemaccording to any one of claims 9-10, wherein the secondary reagentcomprises ferrous iron.
 12. The treatment system according to any one ofclaims 9-11, wherein the reactive solid further comprises an iron oxidemineral.
 13. The treatment system according to claim 12, wherein ironmineral comprises magnetite.
 14. The treatment system according to anyone of claims 1-13, wherein the aqueous suspension comprises a wasteinfluent.
 15. The treatment system according to any one of claims 1-14,wherein the aqueous suspension comprises a toxic material.
 16. Thetreatment system according to any one of claims 1-15, wherein the toxicmaterial is selected from the group consisting of selenium, arsenic,mercury, aluminum, antimony, beryllium, thallium, chromium, cobalt,lead, cadmium, silver, zinc, nickel, molybdenum, nitrates, bromates,iodates, periodates, and borates.
 17. A process for treating an aqueoussuspension, comprising feeding the aqueous suspension to the treatmentsystem according to any one of claims 1-14.
 18. The process according toclaim 17, wherein the process further comprises removing a toxicmaterial from the aqueous suspension.
 19. The process according to claim18, wherein the removing comprises: a) at least one of reacting,adsorbing, and precipitating the toxic material from the aqueoussuspension so as to form removable solids in treated effluent; and b)separating the removable solids from the aqueous suspension.
 20. Theprocess according to claim 19, wherein the removable solids comprise atleast a portion of the toxic material encapsulated in the removablesolids.
 21. A process for treating wastewater comprising a toxicmaterial, comprising exposing the wastewater to a reactive materialsystem so as to remove toxic material from the wastewater, wherein thereactive material system comprises zero valent iron particles andferrous iron, wherein the exposing comprises: a) at least one ofreacting, adsorbing, and precipitating the toxic material from thewastewater so as to form removable solids in treated wastewater, whereinthe removable solids comprise at least a portion of the toxic materialencapsulated in at least a portion of an iron mineral derived from thereactive material system; and b) separating the removable solids fromthe treated wastewater.
 22. A new and improved fluidized bed apparatusfor wastewater treatment comprising a fluidized bed, a fluidizedreactive zone, an internal solid/liquid separating zone in fluidcommunication with said reactive zone, an aerating basin, and a settlingbasin.
 23. The apparatus of claim 22 further comprising control andmetering systems for monitoring and manipulating chemical processeswithin said reactor.
 24. The apparatus of claim 23 further comprising asand filtration bed.
 25. The apparatus of claim 22 further comprising acentral conduit in the fluidized bed reactor to promote convective fluidflow enhancing mixing.
 26. The apparatus of claim 25 further comprisinga motorized stirrer in conjunction with said central conduit configuredso fluid flow within the conduit is down and flow within the fluidizedbed reactor outside the conduit is up.
 27. The apparatus of claim 26further comprising a sand filtration bed.
 28. The apparatus of claim 22further comprising at least one additional fluidized bed apparatusconfigured as stages in series with said first apparatus.
 29. Theapparatus of claim 28 further comprising control and metering systemsfor monitoring and manipulating chemical processes run within saidreactors.
 30. The apparatus of claim 29 wherein the chemical processconditions within different stages are varied to optimize results. 31.The apparatus of claim 22, wherein the fluidized reactive zone comprisesa composition comprising zero valent iron, iron oxide mineral, andferrous iron.
 32. A composition for treating an aqueous suspensioncomprising zero valent iron, iron oxide mineral, and ferrous iron.